Answer is: two atoms of hydrogen are needed to produce a single molecule of water.
Chemical reaction: 2H₂ + O₂ → 2H₂O. /÷2.
H₂ + 1/2O₂ → H₂O.
Beside two atoms of hydrogen, there is also one atom of oxygen in a single molecule of water (H₂O). Atoms of hydrogen and oxygen are connected by polar covalent bonds.